The present invention relates to an aircraft panel and in particular to an aircraft fuselage skin panel comprising a sandwich construction with an outer skin portion of high strength material, an inner skin portion of high strength material and an intermediate core material.
Aircraft, whether military or civil, are subjected to a high degree of stress and need to comply with stated aircraft regulations. Nevertheless, their manufacture must not involve excessive cost. These somewhat contradictory criteria apply in particular to the manufacture of curved skin panels used for fuselage, wings and other part of the aircraft where lightweight materials and simple manufacturing methods are important to keep the cost on an acceptable level.
Today skin panel surfaces on a modern aircraft are supported by one or more of the following methods:                frames        stringers        honeycomb sandwich        
Traditional fuselage skin panels are reinforced with circumferential frames and longitudinal stringers. U.S. Pat. No. 4,498,325 discloses a typical fuselage skin panel with circumferential frames and longitudinal stringers. The combination of frames and stringers is used to give fuselage vessel the required stiffness and the sufficient amount of material to sustain all aircraft loading both from a static and from a fatigue point of view. For the stringer elements in particular, the main purpose is to divide the skin panel surface in an optimal configuration so that deformation of the surface is avoided. The stringers also contribute to preventing cracks from spreading in the skin panel.
Considerable disadvantages exist however in the present design which requires installation of the stringers and frames. Manufacturing of stringers with different geometry is very expensive. Another disadvantage is that stringer installation by riveting is time-consuming and expensive. The method causes a great number of holes through the outer skin, which always increases the potential for corrosion and fatigue problems. Another concern is the intersection where the stringers passes the frames. In the Saab 340 and Saab 2000 project, the frames have cut-outs for the stringers, which decreases the stiffness of the frames. In other solutions, clips are used in the intersection between the stringers and the frames to connect the frame to the outer skin. A problem with this solution is that it increases the complexity in the. Other techniques for stringer installation may reduce some of these disadvantages but are still time-consuming and difficult.
Test with new composite designs are on-going in a number of companies. The composite solutions have a potential to lower the weight significantly because of the unique tailoring capacity. However, there are some major concerns regarding the use of composites in cabin structures that remains to be solved. The epoxy resins, that are used for structural applications, do not meet the Fire, Toxicity and Smoke (FTS) requirements. Furthermore, the crash worthiness requirements are very difficult to meet due to the composites high degree of brittleness. A skin panel manufactured of carbon fiber would not absorb the energy to the same extent as an aluminum panel. A problem with known lightweight designs is that they have proven to be more sensitive to damage and more difficult to repair and maintain. Both these qualities provide for reluctance among the aircraft operators to adopt the new design solutions. This will probably cause more structural break-up and thereby less protection for the passengers unless new composite designs will emerge.
It would therefore be highly desirable to develop new panel structures with reduced weight and increased cost efficiency in the manufacturing.